Carbon fiber precursor fiber bundle, production method and production device therefor, and carbon fiber and production method therefor

ABSTRACT

There are provided a carbon fiber precursor fiber bundle which permits easy bundling of a plurality of small tows into one bundle, is provided with a dividing capability to divide into the original small tows spontaneously at the time of firing, and is suitable for obtaining a carbon fiber that is excellent in productivity and quality, and a production method and a production apparatus of the carbon fiber precursor fiber bundle, and an excellent carbon fiber and a production method thereof. A carbon fiber precursor fiber bundle that has a degree of intermingle of 1 m −1  or less between small tows, consists of substantially straight fibers without imparted crimp, a tow of which straight fibers has a moisture content of less than 10% by mass when housed in a container, and has a widthwise dividing capability to maintain a form of a single aggregate of tows when housed in a container, taken out from the container and guided into a firing step, and to divide into a plurality of small tows in the firing step by the tension generated in the firing step. A production method thereof. A production apparatus of a carbon fiber precursor fiber bundle, comprising an intermingling device that comprises a yarn channel having a flat rectangular section through which a plurality of small tows can pass in a manner adjacent to each other and a plurality of air jet holes disposed with predetermined intervals along the long side direction of the flat rectangle and having the openings thereof in the yarn channel. A carbon fiber using the precursor fiber bundle and a production method thereof.

TECHNICAL FIELD

The present invention relates to a carbon fiber and a production methodthereof. The present invention also relates to a carbon fiber precursorfiber bundle to be used for production of a carbon fiber, and aproduction method and a production apparatus thereof.

BACKGROUND ART

As an acrylonitrile-based precursor fiber for a carbon fiber, for thepurpose of obtaining a carbon fiber having high strength and highelasticity modulus, predominantly a so-called small tow of 3,000 to20,000 filaments which tow scarcely suffers filament breaking and fluffgeneration and is excellent in quality has hitherto been produced; acarbon fiber produced from such a precursor fiber has been used invarious fields such as the aerospace field and the sport field.

A precursor fiber for producing a carbon fiber is subjected to a flameretarding treatment in which, in advance of carbonization, the precursorfiber is heated in an oxidative atmosphere set at 200 to 350° C. Theflame retarding treatment generates a reaction heat, and hence heattends to be stored inside the fiber tow. When excessive heat storageoccurs inside the fiber tow, yarn breaking and fusion bonding betweenfibers tend to be generated. Accordingly, it is necessary to suppressthe heat storage due to the reaction heat as much as possible. For thepurpose of suppressing the heat storage, the thickness of the fiber towfed to a flame retarding oven is inevitably made to be equal to or lessthan a certain thickness; thus, a constraint is imposed on the thicknessof the fiber tow, and accordingly constitutes factors that degrade theproductivity and simultaneously raises the production cost.

For the purpose of solving the above described problems, for example,Patent Document 1 (Japanese Patent Laid-Open No. 10-121325) discloses aprecursor fiber tow for a carbon fiber which tow maintains a form of atow when housed in a container, but has a widthwise dividing capabilityto divide the tow into a plurality of small tows when the tow is takenout, from the container, to be used. For the purpose of producing thefiber tow having this dividing capability, a plurality of spun yarns(fibers) are divided into a plurality of groups each having apredetermined number of yarns, and the plurality of groups are made totravel parallel in this divided condition, made to pass through afiber-making step and a finishing oiling agent imparting step, andtransferred to a crimp imparting step involving a crimper. The crimpimparting carries out bundling of a predetermined number of theplurality of groups into a form of a tow. When the crimp imparting stepis not applied, individual small tows each are made to contain water ina content of 10% or more and 50% or less.

In the bundled form, the yarns at the selvage of each of the yarngroups, each having a small tow form, are made to obliquely cross eachother over approximately 1 mm to be mutually weakly intermingled, andthus, a single tow form made of a plurality of yarn groups ismaintained. The intermingle based on the oblique crossing of the yarnsat the selvage of each of the yarn groups is weak, and hence, when thebundled form is transferred to and used in a carbon fiber producing stepafter having been maintained in a single tow form, easy division intoindividual yarn groups from the selvage is made possible, and thebundled fiber bundle is housed in a container as it is in a conditioncapable of being divided into small tows.

The precursor fiber bundle, having dividing capability, for a carbonfiber housed in a container are divided into the above describedindividual small tows in a dividing step before being guided into theflame retarding oven. This division is to be carried out, for example,by using a grooved roll or a dividing guide bar. The small tows aremutually bundled by being weakly intermingled at the selvages thereof,and hence this division can be extremely easily carried out in such away that breakage or fuzz generation scarcely occurs. The individualtows divided into small tow forms each having a predetermined size orless are guided into the flame retarding step to be subjected to theflame retarding treatment. In this treatment, it is stated that thesmall tows are subjected to the flame retarding treatment as they are inthe divided condition, so that the excess heat storage is not generated,and accordingly the breakage or the fusion bonding between filaments isprevented.

The mechanism for imparting the bundled fiber bundle the dividingcapability into small tows, according to above described Patent Document1, is stated to be based on the intermingling due to oblique crossing ofeach fiber located at the selvages of the small tows; however, with adegree of intermingle of 1 to 10 m⁻¹ at the dividing portions in thesmall tows, when division into small tows is carried out by a dividingmeans in advance of being guided into the flame retarding step, singleyarn breaking is possibly caused and the quality of the carbon fiber ispossibly affected thereby. Furthermore, in Patent Document 1, as meansfor intermingling small tows with each other, there is disclosed only amethod that is based on the imparting of the crimp where a form of a towis maintained by crossing the yarns at the selvages of the individualsmall tows obliquely with each other to weakly intermingle the yarns. Inthe case of such a crimped tow, when the crimped tow is guided as it isinto the flame retarding step involved in a production process of acarbon fiber, it is difficult to uniformly straighten the crimps overthe whole range of the tow to provide a predetermined elongation.Consequently, there may occur unevenness in the basis weight (weight perunit length) and the fineness of the obtained carbon fiber, and thequality of the obtained carbon fiber is possibly thereby affected. Underthese circumstances, a crimp removing means is required in advance ofthe flame retarding step, which increases the equipment space, impedeswork saving and significantly affects the productivity.

On the other hand, in aforementioned Patent Document 1, for the case ofa form of a straight tow without imparted crimp, it is only describedthat the moisture percentage thereof is from 10 to 50%. In other words,there is described only a mechanism that the surface tension due to themoisture bundles the small tows to maintain a form of a single tow. Withsuch moisture percentage, the surface tension due to the water withinthe tow serves to maintain the bent shape formed when folded and housedin a container, and consequently when fed to the production step of thecarbon fiber, the tow is fed as it still has the bent shape and theoblique disposition of the filaments within the tow caused by the bentshape, the quality of the obtained carbon fiber is impaired, orsometimes the bent shape turns into twisted shape, and there is a fearthat excessive heat storage is caused in such twisted portion in theflame retarding step.

Furthermore, irrespective as to whether or not a bundled fiber bundle ismade to pass through a crimper, the bundled fiber bundle is required tobe divided into small tows each having a predetermined thickness beforethe bundled fiber bundle is guided into a firing step after the bundleis taken out from a container; thus, a dividing device is required to beinstalled purposely, which increases the equipment space, impedes worksaving and significantly affects the productivity.

Application of carbon fiber is being expanding over common industrialfields including automobiles, civil engineering, construction andenergy. Accordingly, there are strong demands for supply of large towcarbon fiber high in strength, high in elastic modulus, high in gradeand high in quality as well as large tow carbon fiber lower in price andexcellent in productivity. For example, Patent Documents 2 and 3disclose production methods of a large tow carbon fiber and a carbonfiber precursor fiber bundle; however, the carbon fiber disclosed ineither of these Documents does not attain strength to a sufficientextent, and as affairs now stand, does not reach the strand strength andthe elastic modulus comparable to those of a conventional small towhaving the number of filaments of 12,000 or less.

-   Patent Document 1: Japanese Patent Laid-Open No. 10-121325-   Patent Document 2: Japanese Patent Laid-Open No. 11-189913-   Patent Document 3: Japanese Patent Laid-Open No. 2001-181925

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

An object of the present invention is to provide a carbon fiberprecursor fiber bundle which permits bundling of a plurality of smalltows into a bundled fiber bundle by a simple operation, is provided witha dividing capability to divide into the original small towsspontaneously in a firing step, is low in production cost, is excellentin productivity, scarcely suffers yarn breaking and fuzz generation, andis suitable for obtaining a carbon fiber that is high in grade, high inquality and excellent in strength attainment, and a production methodand a production apparatus of the carbon fiber precursor fiber bundle.

Another object of the present invention is to provide such an excellentcarbon fiber and a production method thereof.

Means for Solving the Problems

The present invention is as described below.

1) A carbon fiber precursor fiber bundle, characterized by:

having 1 m⁻¹ or less of a degree of intermingle between a plurality ofsmall tows based on the hook drop method;

consisting of substantially straight fibers without imparted crimp, atow of which straight fibers has a moisture content of less than 10% bymass when housed in a container; and

having a widthwise dividing capability to maintain a form of a singleaggregate of tows when housed in a container, taken out from thecontainer and guided into a firing step, and to divide into a pluralityof small tows in the firing step by the tension generated in the firingstep.

2) The carbon fiber precursor fiber bundle according to 1), wherein themonofilament fineness is 0.7 dtex or more and 1.3 dtex or less, thenumber of monofilaments of the small tow is 50000 or more and 150000 orless, and the total number of monofilaments in the aggregate of tows is100000 or more and 600000 or less.

3) The carbon fiber precursor fiber bundle according to 1) or 2),wherein the aggregate of tows is formed by intermingling monofilamentsbetween a widthwise end of a small tow and a widthwise end of theadjacent small tow by air flow.

4) The carbon fiber precursor fiber bundle according to any one of 1) to3), wherein the number of monofilaments undergoing adhesion between themonofilaments is 5 per 50,000 of monofilaments or less and the size ofthe crystal region in a direction perpendicular to the fiber axis is1.1×10⁻⁸ m or more.

5) The carbon fiber precursor fiber bundle according to any one of 1) to4), wherein the strength of a monofilament is 5.0 cN/dtex or more, andthe fineness unevenness (CV value) of the monofilament is 10% or less.

6) The carbon fiber precursor fiber bundle according to any one of 1) to5), wherein the oiling agent adhesion unevenness (CV value) along thelengthwise direction is 10% or less.

7) A production method of a carbon fiber precursor fiber bundle,characterized by including:

a coagulation step of forming swollen yarn by extruding an organicsolvent solution of an acrylonitrile-based polymer into an aqueoussolution of dimethylacetamide from a spinning nozzle having a nozzlehole diameter of 45 μm or more and 75 μm or less and the number of holesof 50,000 or more at a coagulated yarn take-up speed/extrusion linearspeed ratio of 0.8 or less;

a wet heat drawing step of wet heat drawing the swollen yarn;

an oiling agent imparting step of imparting a first oiling agent to thewet heat drawn yarn by guiding the wet heat drawn yarn into a first oilbath, and subsequently imparting a second oiling agent in a second oilbath after once squeezing the yarn by use of two or more guides;

a small tow production step of obtaining a small tow by drying,densifying and secondarily drawing the yarn imparted with the first andsecond oiling agents so as to have a total drawing magnification of 5 ormore and 10 or less; and

an aggregate-of-tows production step of obtaining an aggregate of towsby feeding a plurality of the small tows so as to be in parallel andadjacent to each other into an intermingling device that includes a yarnchannel having a flat rectangular section and a plurality of air jetholes which are disposed with a predetermined interval along the longside direction of the flat rectangle and which open into the yarnchannel, and by jetting out air from the air jet holes to interminglethe adjacent small tows with each other.

8) The production method of a carbon fiber precursor fiber bundleaccording to 7), further including an aggregate-of-tows housing step ofhousing the aggregate of tows in a container after the aggregate-of-towsproduction step and a water imparting step of imparting water to thesmall tows before the aggregate-of-tows production step, wherein

the water content of the aggregate of tows in the aggregate-of-towshousing step is set at less than 10% by mass.

9) The production method of a carbon fiber precursor fiber bundleaccording to 7) or 8), further including, before the aggregate-of-towsproduction step, an intra-small-tow intermingling step of interminglingmonofilaments within the small tow with each other by passing the smalltow through an intermingling device, other than the intermingling deviceused in the aggregate-of-tows production step, that includes a yarnchannel having a circular section and an air jet hole which opens intothis yarn channel, and by jetting out air from this air jet hole.

10) The production method of a carbon fiber precursor fiber bundleaccording to 7) or 8), further including, before the aggregate-of-towsproduction step, an intra-small-tow intermingling step of interminglingmonofilaments within the small tow with each other by passing the smalltow through an intermingling device, other than the intermingling deviceused in the aggregate-of-tows production step, that includes a yarnchannel having a flat rectangular section and a plurality of air jetholes which are disposed with a predetermined interval along the longside direction of this flat rectangle and which open into this yarnchannel, and by jetting out air from these air jet holes.

11) The production method of a carbon fiber precursor fiber bundleaccording to 7) or 8), wherein monofilaments within the small tow areintermingled with each other in the aggregate-of-tows production step.

12) The production method of a carbon fiber precursor fiber bundleaccording to 11), wherein the intermingling device, used in theaggregate-of-tows production step, further includes a groove whichextends along the lengthwise direction of the yarn channel and whichopens into the yarn channel at a position where the small tows areadjacent to each other.

13) The production method of a carbon fiber precursor fiber bundleaccording to 9) or 10), wherein:

the intermingling device, used in the aggregate-of-tows production step,further includes a groove which extends along the lengthwise directionof the yarn channel and which opens into the yarn channel at a positionwhere the small tows are adjacent to each other, and the air jet holesopen only into the groove; and

a plurality of the small tows are intermingled with each other, whereinthe filaments within the small tows are intermingled with each other, byfeeding to this intermingling device the plurality of the small towshaving been subjected to the intra-small-tow intermingling step.

14) The production method of a carbon fiber precursor fiber bundleaccording to any one of 7) to 13), further including a step of housingin a container the aggregate of tows obtained in the aggregate-of-towsproduction step after the aggregate of tows has been fed to a gear roll.

15) The production method of a carbon fiber precursor fiber bundleaccording to any one of 7) to 13), further including a step of housingin a container the aggregate of tows obtained in the aggregate-of-towsproduction step after the aggregate of tows has been fed to a nip roll.

16) A production apparatus of a carbon fiber precursor fiber bundle,characterized by including an intermingling device that includes a yarnchannel having a flat rectangular section capable of passing a pluralityof small tows which are adjacent to each other and that includes aplurality of air jet holes which are disposed with a predeterminedinterval along the long side direction of the flat rectangle and whichopen into the yarn channel.

17) The production apparatus of a carbon fiber precursor fiber bundleaccording to 16), further including a groove which extends along thelengthwise direction of the yarn channel and which opens into the yarnchannel at a position where the plurality of small tows are adjacent toeach other.

18) A production apparatus of a carbon fiber precursor fiber bundle,characterized by including:

a first intermingling device that includes a yarn channel having acircular section capable of passing a small tow and that includes one ormore air jet holes for jetting out air into the yarn channel; and

a second intermingling device that includes a yarn channel having a flatrectangular section capable of passing a plurality of small tows whichare adjacent to each other and that includes a plurality of air jetholes which are disposed with a predetermined interval along the longside direction of this flat rectangle and which open into this yarnchannel.

19) A production apparatus of a carbon fiber precursor fiber bundle,characterized by including:

a first intermingling device that includes a yarn channel having a flatrectangular section capable of passing a small tow and that includes oneor more air jet holes for jetting out air into the yarn channel; and

a second intermingling device that includes a yarn channel having a flatrectangular section capable of passing a plurality of small tows whichare adjacent to each other and that includes a plurality of air jetholes which are disposed with a predetermined interval along the longside direction of this flat rectangular shape and which open into thisyarn channel.

20) The production apparatus of a carbon fiber precursor fiber bundleaccording to 18) or 19), wherein the second intermingling device furtherincludes a groove which extends along the lengthwise direction of theyarn channel thereof and which opens into the yarn channel at a positionwhere the plurality of small tows are adjacent to each other.

21) The production apparatus of a carbon fiber precursor fiber bundleaccording to 20), wherein the air jet holes of the second interminglingdevice open only into the groove.

22) The production apparatus of a carbon fiber precursor fiber bundleaccording to 16), wherein the ratio n·D/L of the total fineness nD(dTex) of an aggregate of tows represented by the product between thetotal fineness D (dTex) of the small tow and the number n of the smalltows to be aggregated to the long side dimension L (mm) of the flatrectangular section is 2000 dTex/mm or more and 12,000 dTex/mm or less,and the diameter of each of the air jet holes is 0.3 mm or more and 1.2mm or less.

23) The production apparatus of a carbon fiber precursor fiber bundleaccording to 16), wherein the air jet holes are disposed with an evenpitch, and the pitch is 0.8 mm or more and 1.6 mm or less, and thelength of the yarn channel is 10 mm or more and 40 mm or less.

24) The production apparatus of a carbon fiber precursor fiber bundleaccording to 17) or 20), wherein the groove has a sectional shape of apart of a circle, and the diameter of the circle is 2 mm or more and 10mm or less, and the depth of the groove is 1.5 mm or more and 4 mm orless.

25) The production apparatus of a carbon fiber precursor fiber bundleaccording to 17) or 20), wherein the groove has a trapezoidal sectionalshape, and the dimension of the long side of the trapezoidal groovesection is 2 mm or more and 10 mm or less, and the dimension of theshort side corresponding to groove bottom is 1.5 mm or more and 6 mm orless.

26) A production method of a carbon fiber, characterized in that thecarbon fiber precursor fiber bundle according to any one ofaforementioned 1) to 6) is fed to a flame retarding step, and is firedwhile being divided into small tows by the tension generated in theflame retarding step.

27) A production method of a carbon fiber, characterized in that thecarbon fiber precursor fiber bundle according to any one ofaforementioned 1) to 6) is fed to a carbonization step after a flameretarding step, and is fired while being divided into small tows by thetension generated in the carbonization step.

28) A carbon fiber characterized in that the carbon fiber is produced bythe method according to 27) and the strand strength thereof defined byJIS R7601-1986 is 4100 MPa or more.

29) A production method of a carbon fiber precursor fiber bundle,characterized by including a step of obtaining a single aggregate oftows by disposing a plurality of small tows of carbon fiber precursorfiber so as to be in parallel and adjacent to each other, and byintermingling the adjacent small tows with each other by air flow.

30) The production method of a carbon fiber precursor fiber bundleaccording to 29), wherein, in the step of obtaining an aggregate oftows, the intermingling is carried out by feeding a plurality of thesmall tows so as to be in parallel and adjacent to each other into anintermingling device that includes a yarn channel having a flatrectangular section and a plurality of air jet holes which are disposedwith a predetermined interval along the long side direction of the flatrectangle and which open into the yarn channel, and by jetting out airfrom the air jet holes.

ADVANTAGES OF THE INVENTION

The carbon fiber precursor fiber bundle (aggregate of tows) of thepresent invention can be easily divided into small tows when subjectedto a flame retarding treatment, and hence the heat storage in the fiberbundle can be easily suppressed. Consequently, no constraint is imposedon the thickness of the fiber bundle to be fed to the flame retardingtreatment. Accordingly, there can be obtained a carbon fiber excellentin productivity and low in production cost.

As described above, the fiber bundle is capable of being divided, andhence yarn breaking or fuzz generation is not induced, and the grade andquality of the carbon fiber are not damaged. Accordingly, the use ofsuch a precursor fiber bundle makes it possible to obtain a carbon fiberthat scarcely suffers yarn breaking or fuzz generation, is high ingrade, is high in quality, and is particularly excellent in attainmentof strength.

According to the production method of a carbon fiber precursor fiberbundle of the present invention, there can be suitably produced theabove described small tow or an aggregate of tows. According to theproduction method of a carbon fiber of the present invention, there canbe suitably produced such an excellent carbon fiber as described above.

The use of the production apparatus of a carbon fiber precursor fiberbundle of the present invention makes it possible to suitably producethe above described aggregate of tows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic process drawing illustrating an example of aproduction process of a-carbon fiber precursor fiber bundle in whichprocess intermingling is carried out by air jet;

FIG. 2 is a schematic view illustrating an example of structure of afirst intermingling device that carries out intermingling within a smalltow by air jet, wherein (a) is a front sectional view as viewed from thedirection of travel of the fiber bundle, (b) is a side sectional view,and (c) is a top sectional view;

FIG. 3 is a schematic view illustrating an example of structure of asecond intermingling device that carries out intermingling between smalltows by air jet, wherein (a) is a front sectional view as viewed fromthe direction of travel of the fiber bundle and (b) is a side sectionalview;

FIG. 4 is a schematic process drawing illustrating another example of aproduction process of a carbon fiber precursor fiber bundle in whichprocess confounding is imparted by air jet;

FIG. 5 is a schematic view illustrating an example of structure of asecond intermingling device that has grooves and imparts confoundingbetween small tows, wherein (a) is a front sectional view as viewed fromthe direction of travel of the fiber bundle and (b) is a side sectionalview;

FIG. 6 is a schematic view illustrating an example of structure of asecond intermingling device that has air jet holes only inside thegrooves and imparts confounding between small tows, wherein (a) is afront sectional view as viewed from the direction of travel of the fiberbundle and (b) is a side sectional view;

FIG. 7 a schematic view illustrating another example of a secondintermingling device that has air jet holes only inside the grooves andimparts confounding between small tows, wherein (a) is a front sectionalview as viewed from the direction of travel of the fiber bundle and (b)is a side sectional view; and

FIG. 8 is a schematic partial view illustrating the roundness R at anangular groove portion.

DESCRIPTION OF SYMBOLS

-   1: small tow-   2: spray-   3: first intermingling device-   4, 9, 20, 21, 26: yarn channel-   5: upper nozzle-   6: lower nozzle-   5 a, 6 a, 10 a, 11 a: compressed air introduction part-   5 b, 6 b, 10 b, 11 b, 18 b, 19 b, 22 b, 23 b, 27 b, 28 b: air jet    hole-   7: driving roll-   8, 17, 24, 25: second intermingling device-   12: aggregate of tows-   13: gear roll-   14: chute-   15: container-   16: touch roll-   18 c, 19 c, 22 c, 23 c, 27, 28 c: groove-   30: roundness R at an angular groove portion

BEST MODE FOR CARRYING OUT THE INVENTION

The above described problems can be solved by a carbon fiber precursorfiber bundle of the present invention that has 1 m⁻¹ or less of a degreeof intermingle between a plurality of small tows based on the hook dropmethod, that consists of substantially straight fibers without impartedcrimp, a tow of which straight fibers has a moisture content of lessthan 10% by mass when the tow is housed in a container, and that has awidthwise dividing capability to maintain a form of a single aggregateof tows when housed in a container, taken out from the container andguided into a firing step, and to divide into a plurality of small towsin the firing step by the tension generated in the firing step.

The carbon fiber precursor fiber bundle of the present inventionmaintains a form of a single tow as a mutual aggregate of a plurality ofsmall tows without impairing the grade thereof, and can be divided,without entangling between small tows, due to the tension generated whenfired without installing a dividing guide or the like, while maintaininga form of a single tow when taken out from a container.

In the carbon fiber precursor fiber bundle, the monofilament fineness ispreferably 0.7 dtex or more and 1.3 dtex or less, the total number offilaments is preferably 100000 or more and 600000 or less, and thenumber of filaments in each of the small tows is preferably 50000 ormore and 150000 or less. When the monofilament fineness is 0.7 dtex ormore, it is easy to stably spin a stock yarn, for a carbon fiberprecursor fiber, such as an acrylic fiber yarn. When the monofilamentfineness is 1.3 dtex or less, a high performance carbon fiber can beobtained while the sectional double structure is suppressed so as not tobe remarkable. When the total number of filaments in the carbon fiberprecursor fiber bundle is 100000 or more, firing can be carried out withan excellent productivity while suppressing a possibility of thedecrease in the number of the actually fired small tows in the firingstep. When the total number of filaments in the carbon fiber precursorfiber bundle is 600000 or less, it is easy to house a carbon fiberprecursor fiber bundle having a desired length in a container. When thenumber of filaments of a small tow is 50000 or more, there can besuppressed a possibility that an increased division-number impedes theattainment of the dividing capability in the firing step, and there canalso be suppressed a possibility that the forming efficiency is loweredbecause the small tow is thin. When the number of filaments of a smalltow is 150000 or less, the heat storage due to the reaction heat can besuppressed in the flame retarding step, and generation of yarn breaking,fusion bonding or the like can be excellently suppressed.

The number of the adhered monofilaments is preferably as small aspossible, from the viewpoint of suppressing the generation of fuzz,bundle breakage and the like in the subsequent flame retarding step,pre-carbonization step and carbonization step due to the adhesionbetween the monofilaments, and from the viewpoint of preventing thedegradation of the strand strength. From these viewpoints, the number ofthe monofilaments undergoing adhesion between the monofilaments,constituting a carbon fiber precursor fiber bundle, is preferably 5 per50000 of monofilaments or less. And the size of the crystal region in adirection perpendicular to the fiber axis is preferably 110 Å (1.1×10⁻⁸m) or more.

The monofilament strength of the carbon fiber precursor fiber bundle ispreferably 5.0 cN/dtex or more, more preferably 6.5 cN/dtex or more, andfurthermore preferably 7.0 cN/dtex or more. When the monofilamentstrength is 5.0 cN/dtex or more, there can be excellently prevented apossibility that increased occurrence frequency of fuzz generation dueto single yarn breaking in the firing step degrades the firing steppassage performance, and thus a carbon fiber excellent in strength canbe obtained.

The fineness unevenness (CV value) of the monofilaments constituting theprecursor fiber bundle is preferably 10% or less, more preferably 7% orless, and furthermore preferably 5% or less. When this value is 10% orless, there can be excellently prevented yarn breaking and twiningtrouble in the spinning step and the firing step.

The oiling agent adhesion unevenness (CV value) of the precursor fiberbundle along the lengthwise direction is preferably 10% or less, andmore preferably less than 5%. When this value is 10% or less, generationof adhesion and fusion bonding can be excellently prevented in thespinning step, and consequently, troubles such as single yarn breakingor bundle breakage can be excellently prevented. The oiling agentadhesion unevenness falling within the above described range ispreferable for the carbon fiber to be obtained from the view points ofthe quality and performances (particularly, strand strength). For thepurpose of obtaining a high-quality and high-performance carbon fiberprecursor yarn bundle and a high-quality and high-performance carbonfiber, it is preferable to make an oiling agent to adhere as evenly aspossible irrespective of the total fineness of a small tow or a largetow.

According to the present invention, a carbon fiber precursor fiberbundle can be obtained by obtaining an aggregate of tows by disposing aplurality of the small tows of the carbon fiber precursor fiber so thatthe small tows are in parallel and adjacent to each other, an bycarrying out confounding between the adjacent tows using air flow.According to this method, there can be formed an aggregate of tows,without imparting crimp thereto, provided with a dividing capability todivide the aggregate into the original small tows spontaneously in thefiring step (flame retarding step, carbonization step).

When obtaining an aggregate of tows, the confounding may be carried outby feeding the plurality of the small tows, so that the small tows arein parallel and adjacent to each other, into an intermingling devicethat includes a yarn channel having a flat rectangular section and aplurality of air jet holes which are disposed with a predeterminedinterval along the long side direction of the flat rectangle and whichopen into the yarn channel, and by jetting out air from the air jetholes.

The carbon fiber precursor fiber bundle of the present invention may beproduced, for example, by the following method. That is, a spinningsolution consisting of an acrylonitrile-based polymer and an organicsolvent is extruded into an aqueous solution of dimethylacetamide from aspinning nozzle having a nozzle hole diameter of 45 μm or more and 75 μmor less and the number of holes of 50,000 or more at a “coagulated yamtake-up speed/extrusion linear speed” ratio of 0.8 or less, and thus aswollen yarn is obtained. When the number of the holes is 50000 or more,the productivity can be made excellent. The number of the holes ispreferably 150000 or less from the viewpoint of suppressing generationof yarn breaking, fusion bonding or the like, due to the heat storagebased on the reaction heat, in the flame retarding step and further fromthe viewpoint of enabling the downsizing of the spinning nozzle pack andthereby increasing the number of production spindles per an apparatus.

When the “coagulated yarn take-up speed/extrusion linear speed” ratio is0.8 or less, yarn breaking from the nozzle can be prevented tofacilitate stable spinning. This ratio is preferably 0.2 or more fromthe viewpoint of performing uniform coagulation and thereby suppressinggeneration of fineness unevenness.

Subsequently, the swollen yarn is wet heat drawn, then guided into afirst oil bath to be imparted with a first oiling agent, once squeezedby use of two or more guides, subsequently imparted with a second oilingagent in a second oil bath, and then dried, densified and secondarilydrawn so as to have a total drawing magnification of 5 or more and 10 orless; and thus an acrylonitrile-based precursor fiber bundle can beobtained. It is to be noted that the total drawing magnification means adrawing magnification attained by all the drawing operations applied inthe course of obtaining a precursor fiber bundle from a spinningsolution; thus, in the above described case where solely applied are awet heat drawing and a secondary drawing, the total drawingmagnification means the product of the magnifications involved in thesetwo drawings.

Examples of the organic solvent for the acrylonitrile-based polymer tobe used in the spinning solution include dimethylacetamide,dimethylsulfoxide and dimethylformamide. Among these, dimethylacetamideis preferably used because it scarcely deteriorates due to hydrolysis ofthe solvent and gives excellent spinning properties.

As the spinneret for extruding a spinning solution, there may be used aspinneret having nozzle holes of 45 μm or more and 75 μm or less in holediameter which spinneret is suitable for producing a monofilament of anacrylonitrile-based polymer of 0.7 dtex or more and 1.3 dtex or less inmonofilament fineness. The use of such a nozzle small in hole diametermakes it easy to reduce (to 0.8 or less) the ratio of (the coagulatedyarn take-up speed)/(the extrusion linear speed of the spinning solutionfrom the nozzle), and to maintain excellent spinning properties.

A swollen yarn taken out from a coagulation bath is made higher in fiberorientation by a subsequent wet heat drawing. The wet heat drawing iscarried out by drawing in hot water a swollen fiber bundle which is in aswollen state.

The degree of swelling of the swollen fiber bundle, after application ofwet heat swelling and before drying, is preferably made to be 100% bymass or less. The fact that the degree of swelling of the swollen fiberbundle, after application of wet heat swelling and before drying, is100% by mass or less means that the surface part and the interior partof the fiber undergo uniform orientation. After the coagulation of acoagulated yarn in a coagulation bath is made uniform by decreasing “thecoagulated yarn take-up speed/the extrusion linear speed of a spinningsolution from the nozzle” in the production of a coagulated yarn in acoagulation bath, the coagulated yarn is wet heat drawn, and thusuniform orientation can be attained even inside the yarn. In this way,the degree of swelling of the fiber bundle before drying can be made tobe 100% by mass or less.

According to the present invention, in a production method of a carbonfiber precursor fiber bundle, there can be obtained a fiber bundle thatmaintains a form of a single aggregate of tows, by imparting mutualconfounding between the filaments in each of small tows and the mutualbundling property between the small tows through carrying out by air jetthe mutual confounding of the filaments in each of the small tows andthe mutual confounding of the small tows. In this case, the widthwiseends of each of the small tows are preferably mutually intermingled tothereby maintain a tow form. The confounding between the small tows ispreferably weaker than the confounding between the filaments in each ofthe small tows. Further, in this case, the widthwise ends of the smalltows are not necessarily required to overlap with each other, but thewidthwise ends of the small tows are preferably adjacent to each otherso as to be in contact with each other.

In the present invention, water is imparted if needed, and when placedin a predetermined vessel, the moisture content of each of the smalltows is preferably set at less than 10% by mass, and more preferably at0.5% by mass or more and 5% by mass or less. By setting the impartedwater content at 0.5% by mass or more, static electricity generation canbe suppressed and the handlability can thereby be made excellent. Bysetting the imparted water content at less than 10% by mass, it becomespossible to eliminate a phenomenon that the tow width is made unstableby the remaining bent shape of the folding portion of the tow which bentshape has been formed by the weight of the tow itself at the time ofhousing or by being housed in the container in a state compressed by apress, and at the same time, the transport efficiency is increased andthe economical efficiency is thereby increased.

Such a carbon fiber precursor as described above may be produced by aproduction method of a carbon fiber precursor fiber bundle including anaggregate-of-tows production step in which a plurality of small tows arejoined together by air jet to each other in a state of being in parallelto each other. The fundamental configuration of the method resides in aproduction method of a carbon fiber precursor fiber bundle in whichmethod a plurality of small tows, each made into a yarn in a state ofbeing divided from each other, are housed in a container after thewidthwise ends of the small tows have been loosely intermingled witheach other. Preferably, when housed in a container, the small tows aretaken up with a gear roll, a nip roll or the like, and are housed asthey are in a container because the form of the fiber bundle is therebymade more stable.

Small tows adjacent to each other may be intermingled with-each other byfeeding a plurality of small tows, so that the small tows are adjacentand in parallel to each other, to a yarn channel of an interminglingdevice that includes a plurality of air jet holes disposed in the yarnchannel having a flat rectangular sectional shape with a predeterminedinterval along the long side direction of this flat rectangular section,and by jetting out air from the air jet holes. Here, it should be notedthat, in the present specification, the intermingling device that isused for producing an aggregate of tows by intermingling the small towswith each other is referred to as a second intermingling device, and theintermingling device, to be described below, that carries outintermingling inside a small tow is referred to as a first interminglingdevice.

Before intermingling small tows with each other, the width of a smalltow itself can be controlled and the bundling property can be impartedto the small tow itself by beforehand passing the small tows through thefirst intermingling device. In this case, a desired tow width and adesired bundling property can be imparted to the small tow by passingthe small tow through an air intermingling device that includes a yarnchannel having a circular section and an air jet hole which opens intothis yarn channel having the circular section and by jetting out airfrom the air jet hole, or by passing the small tow through an airintermingling device that includes a yarn channel having a flatrectangular section and a plurality of air jet holes which open into theyarn channel with a predetermined interval along the long side directionof this flat rectangular section and by jetting out air from the air jetholes.

In this case, the control of the width of the small tow and thesecurement of the bundling property of the small tow are carried outbeforehand with the first intermingling device in a manner dedicated tothe small tow; subsequently, for the purpose of bundling to unify thesmall tows with each other, the small tows are fed, so that the smalltows are adjacent and in parallel to each other, to the secondintermingling device, including a yarn channel having a flat rectangularsection, that is disposed next to the first intermingling device, andthus the plurality of the small tows adjacent to each other, having beenbeforehand intermingled, can be bundled with each other into a unifiedarticle.

Alternatively, in the present invention, without particularlyintermingling a small tow itself in advance, it is possible tointermingle the filaments in each of the adjacent small tows with eachother and intermingle the adjacent small tows with each othersimultaneously. In other words, in an aggregate-of-tows production step,the filaments within a small tow may be intermingled with each other. Inthis case, it is possible to intermingle the filaments within each ofthe small tows with each other and intermingle the adjacent small towswith each other simultaneously by feeding a plurality of small towswhich have not been intermingled, so that the small tows are adjacentand in parallel to each other, to an intermingling device that includesa plurality of air jet holes disposed with predetermined intervals alongthe long side direction of a flat rectangular section of a yarn channelhaving a flat rectangular yarn channel sectional shape, and by jettingout air from these air jet holes.

In the yarn channel shape, having a flat rectangular section, to be usedfor intermingling the filaments with each other within a small tow, thedimension thereof may vary depending on the total fineness of the smalltow, but is such that the dimension along the height direction, namely,the short side of the flat rectangular section is preferably 1 mm ormore and 5 mm or less, and more-preferably 2 mm or more and 4 mm orless. When the height is small, namely, the thickness of a tow islimited, the movement of the filaments due to air flow is restricted.This is disadvantageous from a view point that the degree of intermingletends to decrease. On the contrary, when this dimension is large, thethickness of a tow becomes large although the thickness also depends onthe long side dimension. This is disadvantageous from a view point thatthe degree of intermingle tends to decrease.

An intermingling device has, for example, a structure shown in FIG. 2which device can be used for intermingling filaments within a small towwith each other, and includes a yarn channel having a flat rectangularsectional shape and a plurality of air jet holes disposed with apredetermined interval along the long side direction of the flatrectangular sectional shape. As for the long side dimension, there is apreferable range from the viewpoint of controlling the total fineness ofthe small tow and the tow width. The numerical value representing such apreferable range is the value of the ratio D/L of the total fineness D(dTex) of the small tow 1 to the long side dimension L (mm) of the flatsectional yarn channel 4, and the ratio value is preferably 2000 dTex/mmor more and 12000 dTtex/mm or less. In this connection, the hole size(diameter) of each of the air jet holes 5 b and 6 b is preferably 0.3 mmor more and 1.2 mm or less, and more preferably 0.5 mm or more and 1.0mm or less.

Further, from the viewpoint of obtaining uniform intermingling, thedisposition of the air jet holes is preferably such that the holes aredisposed with an even pitch of 0.8 mm or more and 1.6 mm or less. Thelength of the yarn channel 4, namely, the length of the interminglingdevice is preferably 10 mm or more and 40 mm or less. When this lengthexceeds 40 mm, it is disadvantageous from the viewpoint that there tendsto occur disturbance or fluttering of tows probably ascribable to thedisturbance of the ejected air flow at each of both ends of the yarnchannel, and the intermingling tends to be nonuniform.

For the purpose of intermingling adjacent small tows with each other, aplurality of small tows may be fed, so that the small tows are adjacentto each other, to an intermingling device shown in FIG. 3 that includesa flat rectangular sectional shape of a yarn channel and a plurality ofair jet holes disposed in this yarn channel with a predeterminedinterval along the long side direction of the flat rectangular shape. Asfor the long side dimension L of the flat rectangle, there is naturallya preferable range for the purpose of controlling the tow width by thetotal fineness of the small tows and the number of filaments (fibers) tobe aggregated, namely, with respect to the total fineness of theaggregate of tows.

Specifically, the above range means the value of a ratio n·D/L of thetotal fineness nD (dTex) of an aggregate of tows represented by theproduct between the total fineness D (dTex) of the small tow and thenumber n of small tows to be aggregated to the long side dimension L(mm), and the value of the ratio is preferably 2000 dTex/mm or more and12000 dTex/mm or less. In this connection, the hole diameter of each ofthe air jet holes is preferably 0.3 mm or more and 1.2 mm or less, andmore preferably 0.5 mm or more and 1.0 mm or less.

Further, from the viewpoint of uniform intermingling, the disposition ofthe air jet holes is preferably such that the holes are disposed with aneven pitch of 0.8 mm or more and 1.6 mm or less. The pitch for the airjet holes is preferably 0.8 mm or more from the viewpoint of suppressingthe generation of disturbance or fluttering of tows due to ejected airflow, and is preferably 1.6 mm or less from the viewpoint of suppressingthe generation of intermingling unevenness due to the revolution ofmonofilaments within a tow.

The length of the yarn channel, namely, the length of the interminglingdevice is preferably 10 mm or more and 40 mm or less. When this lengthexceeds 40 mm, it is disadvantageous from the viewpoint that there tendsto occur disturbance or fluttering of tows probably ascribable to thedisturbance of the ejected air flow at each of both ends of the yarnchannel, and the intermingling tends to be nonuniform.

Further, in an intermingling device that includes a plurality of air jetholes disposed, in a yarn channel having a flat rectangular sectionalshape of the yarn channel to intermingle adjacent small tows with eachother, with a predetermined interval along the long side direction ofthe flat rectangular shape, there may be formed, as shown in FIG. 5, agroove extending along the lengthwise direction of the yarn channel atthe position of the adjacent ends of the small tows to be aggregated.The provision of such a groove leads to formation of a space permittingfree movement of the filaments at the adjacent ends of the small tows tobe intermingled with each other in the flat rectangular sectional yarnchannel, and hence the small tows adjacent to each other can beefficiently intermingled with each other.

The sectional shape (across the fiber bundle passage direction) of sucha groove may be a shape of a part of a circle such as a semicircle or atrapezoidal shape as shown in FIG. 5. In the case of a semicirculargroove, if a portion in contact with filaments is angular, there is apossibility such an angular portion damages the tow. For the purpose ofavoiding such a fear, it is preferable to provide a roundness R to theangular groove portion facing the yarn channel. It is more preferable touse a trapezoidal groove in place of a groove having a sectional shapeof a part of a circle. Also in the case of a trapezoidal groove, it ispreferable to provide a roundness R to the angular groove portion facingthe yarn channel. FIG. 8 shows an example in which a roundness R 30 isprovided to each of the angular portions facing the yarn channel in thetrapezoidal groove 18 c shown in FIG. 5. A similar roundness R may alsobe provided to the trapezoidal groove 19 c provided on the downside ofthe yarn channel.

When the groove section is a part of a circle such as a semicircle, thesize of a groove is preferably approximately 2 mm or more and 10 mm orless and more preferably 3 mm or more and 8 mm or less in terms of thediameter of the circle, and the depth of the groove is preferablyapproximately 1.5 mm or more and 4 mm or less. In the case of atrapezoidal groove, the long side dimension of a trapezoidal groovedisposed on the long side portion of a flat yarn channel is preferably 2mm or more and 10 mm or less and more preferably 3 mm or more and 8 mmor less, and the dimension of the short side corresponding to the groovebottom is preferably approximately 1.5 mm or more and 6 mm or less. Theends of small tows adjacent to each other are intermingled with eachother in a groove, and hence an air jet hole is disposed so as to ejectair into the interior of the groove. From the viewpoint of stabletraveling of small tows and uniform intermingling, the disposition ofsuch a hole is preferably such that holes are disposed in a bilaterallysymmetric manner within the groove shape, or a hole is disposed on thecentral line in the bottom of the groove. This is conceivably becausethe provision of a groove in the yarn channel probably makes smooth thedischarge of the ejected air from the intermingling device; there isalso obtained an effect to stabilize the form and the traveling of thesmall tows traveling in a manner adjacent to each other on the entranceside of the intermingling device.

Further, in the present invention, a nozzle having such a groove asdescribed above may be a nozzle in which an air jet hole is disposedonly in the groove as shown in FIG. 6. In this way, it becomes easy tomore loosely intermingle the small tows with each other as compared withthe intermingling of the filaments within a small tow to maintain asingle tow form.

The carbon fiber precursor fiber bundle obtained as described abovepreferably has a fiber degree of intermingle between the small towsbased of less than 1 m⁻¹ on the hook drop method. By setting the fiberdegree of intermingle at less than 1 m⁻¹, it becomes easy to carry outdivision into small tows only by the tension generated in the flameretarding step or the carbonization step involved in the carbon fiberproduction step, a dividing guide bar or the like becomes unnecessary,damaging of tows and yarn breaking caused by scratching are therebysuppressed, and hence it becomes easy to make excellent the grade of theobtained carbon fiber.

In the present invention, small tows may be fed to an intra-small-towintermingling device with regulating the yarn channel of a plurality ofsmall tows so that the side ends of the small tows adjacent to eachother may be in contact with each other by using a curved guide or thelike, after intermingling the monofilaments within a small tow with eachother.

The carbon fiber precursor fiber bundle bundled as described above maybe once housed in a container as described before, and thereafter, maybe taken out from the container to be guided into the flame retardingstep or the carbonization step. When taken out, a form of an aggregateof tows is not collapsed. The carbon fiber precursor fiber bundlespontaneously divides into a plurality of small tows by the tensiongenerated during the firing step. Thus stable firing can be carried outto yield a high-quality carbon fiber.

A carbon fiber obtained in the present invention is a carbon fiberhaving a strand strength (JIS R7601-1986) of 4100 MPa or more,preferably 4400 MPa or more, and more preferably 4900 MPa or more. Whenthe strand strength is 4100 MPa or more, it becomes easy to apply such acarbon fiber to common industrial fields requiring high strengthcomparable with that of small tow.

The carbon fiber of the present invention can be obtained by firing theabove described acrylonitrile-based precursor fiber bundle on the basisof the methods well known in the art. Preferred among these method is amethod in which a carbon fiber precursor fiber bundle is subjected to aflame retarding treatment continuously while shrinkage thereof is beinglimited in a flame retarding oven having zones, where temperature ofeach zone is controlled at from 220° C. to 250° C. to make a temperaturegradient from a lower temperature to a higher temperature over the oven;thus a flame retardant fiber yarn having a density of approximately 1.36g/cm³ is obtained; then a carbonization treatment is carried out forfrom 1 to 5 minutes, while shrinkage is being limited, in acarbonization furnace having a nitrogen atmosphere with a temperaturedistribution extending from 300° C. to 700° C.; and subsequently acarbonization treatment is carried out for from 1 to 5 minutes, whileshrinkage is being limited, in a carbonization furnace having a nitrogenatmosphere with a temperature distribution extending from 1,000° C. to1,300° C.

(Measurement Method of the Number of Adhered Monofilaments)

Identification of the adhesion between monofilaments can be carried outas follows: a precursor fiber bundle is cut approximately to 5 mm anddispersed in 100 mL of acetone, the dispersion is stirred for 1 minuteat 100 rpm and then filtered with a black paper filter and the number ofadhered monofilaments is measured.

(Measurement Method of the Size of a Crystal Region)

The size of a crystal region can be measured as follows. Anacrylonitrile-based precursor fiber bundle is cut to 50 mm in length; 30mg of the cut fibers is accurately weighed out as a sample; the cutsample fibers are aligned by pulling in such a way that the fiber axesof the cut sample fibers are accurately parallel to each other; then,the cut sample fibers are shaped by use of a sample shaping jig into afiber sample bundle having a 1 mm width and a uniform thickness. Thefiber sample bundle is impregnated with a methanol solution of vinylacetate, coagulated in such a way that the form of the bundle is notcollapsed, and then fixed on the sample stage of a wide angle X-raydiffractometer. As the X-ray source, a CuKα radiation (a Ni filter isused) X-ray generator manufactured by Rigaku Corporation is used. Withaid of a goniometer manufactured by Rigaku Corporation a diffractionpeak at around 2θ=17° corresponding to the plane indexes (100) ofgraphite is detected on the basis of the transmission method with ascintillation counter. The output is at 40 kV-100 mA in the measurement.From the half width of the diffraction peak, the size La of a crystalregion is derived by using the following formula:La=Kλ/(β₀ cos θ)wherein K denotes the Scherrer constant of 0.9, λ denotes the wavelengthof the used X-ray (1.5418 Å because here is used CuKα ray), θ denotesthe Bragg diffraction angle, β₀ denotes the true half width, andβ₀=β_(E)−β₁ where β_(E) is the apparent half width and β₁ is theapparatus constant and is, in this case, 1.05×10⁻² rad.

(Measurement Method of the Monofilament Strength)

The monofilament strength can be measured as follows. A monofilamentautomatic tensile strength/elongation tester (manufactured by OrientecCo., Ltd., trade name: UTM II-20) is used. A monofilament affixed on aboard is secured in a load cell with a chuck and subjected to a tensiletest at a rate of 20.0 mm/min and thus a strength and an elongation aremeasured.

(Measurement Method of the Fineness Unevenness (CV Value) ofMonofilaments)

The fineness unevenness (CV value) of a monofilament can be measured asfollows. A fiber of an acrylonitrile-based polymer to be measured isinserted into a vinyl chloride resin tube of 1 mm in inside diameter,and then the tube is cut into a round slice with a knife to prepare asample. The sample is adhered on a SEM sample stage in such a way thatthe fiber section of the acrylonitrile-based polymer can be seen fromupside, and then Au is sputtered on the sample to a thickness ofapproximately 10 nm. The sample thus obtained is subjected to anobservation of the fiber section with a scanning electron microscope(manufactured by Philips, trade name: XL20 scanning electron microscope)under the conditions that the acceleration voltage is 7.00 kV and theoperating distance is 31 mm, and thus, the fiber sectional area of themonofilament was measured randomly for approximately 300 monofilamentsto derive the monofilament fineness.CV value (%)=(Standard deviation/mean fineness)×100wherein the standard deviation and the mean fineness are the standarddeviation and the mean value of the above described finenesses.

(Measurement of the Lengthwise Directional Adhesion Unevenness of anOiling Agent)

The lengthwise directional adhesion unevenness of an oiling agent can bemeasured as follows. Along the lengthwise direction of a precursor yarn,sampling is successively made with N (the number of samples)=10, and thesamples are subjected to the measurement of the oiling agent adhesionunevenness with a wavelength dispersive fluorescent X-ray analyzer(manufactured by Rigaku Denki Kogyo Co., Ltd., trade name: ZSXmini).

(Measurement Method of Degree of Swelling)

The degree of swelling can be derived by using the mass w after theremoval of the adhered liquid with a centrifugal separator (3000 rpm, 15minutes) from a fiber bundle in a swollen state and the mass w₀ afterdrying the thus treated bundle with a hot air dryer at 105° C. for 2hours, on the basis of the following formula: the degree of swelling (%by mass)=(w−w₀)×100/w₀.

(Measurement Method of Moisture Content)

The moisture content is a value (% by mass) obtained by using the mass wof a carbon fiber precursor fiber bundle in a wet state and the mass woafter drying the bundle with a hot air dryer at 105° C. for 2 hours, onthe basis of the following formula: (w−wo)×100/wo.

(Evaluation Method of Degree of Intermingle)

The evaluation is made with a hook drop method. A tow is hooked with aload, at one end thereof, of 10 g/3000 denier (10 g/330 Tex) while theoriginal form of the tow is being maintained. A string of wire of 1 mmin diameter crooked perpendicularly at a position of 20 mm away from thestring tip is connected to a 10 g weight. The weight is hooked betweentows and is allowed to fall freely. The falling distance thus obtainedis represented by X m and the degree of intermingle is given byDegree of intermingle=1/X.The measurement is repeated 30 times for a sample and 20 middlemeasurement values out of 30 values are adopted to derive an averagevalue to be used.

EXAMPLES

Hereinafter, specific description will be made on the production of asmall tow of the carbon fiber precursor fiber to be a target of thepresent invention on the basis of representative examples.

Example 1

Production Method (I) of a Small Tow

Acrylonitrile, acrylamide and methacrylic acid were subjected tocopolymerization based on aqueous suspension polymerization in thepresence of ammonium persulfate-ammonium hydrogen sulfite and ironsulfate, and there was obtained an acrylonitrile-based polymer composedof acrylonitrile unit/acrylamide/methacrylic acid unit=96/3/1 (massratio). The acrylonitrile-based polymer was dissolved indimethylacetamide to prepare a 21% by mass spinning solution.

The spinning solution was extruded through a spinneret of 50,000 in holenumber and 45 μm in hole diameter into a coagulation bath composed of an60% by mass aqueous solution of dimethylacetamide at 35° C. to prepare acoagulated yarn, and the coagulated yarn was taken up at a take-up speedof 0.40 times the extruding speed of the spinning solution.

Then, the fiber thus obtained was subjected to wet heat drawing at amagnification of 5.4 while simultaneously carrying out washing in hotwater, guided into a first oil bath of an aminosilicon-based oilingagent of 1.5% by mass to be imparted with the first oiling agent, oncesqueezed with a few guides, and subsequently imparted with a secondoiling agent in a second oil bath of an aminosilicon-based oiling agentof 1.5% by mass. The fiber was dried with a hot roll, and secondarilydrawn between hot rollers at a magnification of 1.3 to result in a totaldrawing magnification of 7.0. Thereafter, the moisture content of thefiber was regulated with a touch roll to yield a carbon fiber precursorfiber bundle (small tow) having a monofilament fineness of 1.2 dtex.

Three small tows 1, the carbon fiber precursor fiber bundles, obtainedas described above were used. As shown in FIG. 1, with a spray 2,ion-exchanged water was imparted to each of the small tows, andthereafter, the three small tows 1 were fed respectively to three firstintermingling devices 3 shown in FIG. 2 each of which givesintermingling to each small tow. Each of the intermingling devices 3 forthe respective small tows 1 had a structure shown in FIG. 2. In otherwords, the first intermingling device 3 had an upper nozzle 5 and alower nozzle 6 having a flat rectangular yarn channel 4 passing throughin the traveling direction of the tow in the central portion of thenozzles. The upper and lower nozzles 5 and 6 each had a verticallysymmetric structure in a manner sandwiching the yarn channel 4,respectively had the compressed air introduction parts 5 a and 6 a, andrespectively had many air jet holes 5 b and 6 b that werecommunicatively connected respectively to the compressed airintroduction parts 5 a and 6 a and had the openings thereof on thefacing sides along the air introduction directions. The yarn channelwidth of the yarn channel 4 was 8 mm, the yarn channel height was 3 mm,and the yarn channel length (in the traveling direction of the smalltow) was 20 mm. The jet opening diameter of each of the air jet holes 5b and 6 b was 1 mm, the disposition pitch thereof was set at 1.5 mm. Thepressure of the fed air was set at 50 kPa-G (G indicating the gaugepressure).

The three small tows 1 respectively intermingled with the three firstintermingling devices 3 were pulled and aligned, once made to passthrough a driving roll 7, and fed to a second intermingling device 8 tointermingle the adjacent small tows 1 with each other. The secondintermingling device 8 had a structure shown in FIG. 3. The fundamentalstructure thereof was similar to that of the first intermingling device3 dedicated to the small tow; however, because the small tows 1 wererespectively intermingled beforehand, the width of the yarn channel 9was larger by a factor of 3 or more than that of the first interminglingdevice, and the height of the yarn channel thereof was set to beslightly lower than that of the first intermingling device 3.

Incidentally, in the second intermingling device 8, the yarn channelwidth was set at 24 mm, the yarn channel height was set at 2.5 mm, theyarn channel length was set at 20 mm, the opening diameter of each ofthe air jet holes 10 b and 11 b was set at 0.5 mm, the disposition pitchthereof was set at 0.8 mm, and the pressure of the air to be fed to thecompressed air introduction parts 10 a and 11 a was set at 300 kPa-G.One carbon fiber precursor fiber bundle thus obtained was fed to a gearroll 13 to be taken up, and then placed as it was in a container 15through a chute 14. When housed in the container 15, the carbon fiberprecursor fiber bundle 12 had a form of a single tow (aggregate of tows)in which three small tows were aggregated. At this time, namely, afterbeing housed in the container, the moisture content of the carbon fiberprecursor fiber bundle 12 was 2% by mass. By the gear roll 13 used whenplaced in the container 15, wave was imparted to the obtained tow. Thedistance between a crest of the wave and an adjacent crest was 25 mm.The degree of intermingle of the carbon fiber precursor fiber bundle 12thus obtained was evaluated and found to be less than 1 m⁻¹. (Theevaluation was carried out with a test length of 1 m, and any of the 10g loads fell with a falling distance of 1 m or more, so that themeasurement was impossible.)

The carbon fiber precursor fiber bundle 12 thus obtained was taken outfrom the container 15, and was fed to the flame retarding step withoutdividing into small tows, subjected to a flame retarding treatment for70 minutes, and further subjected to a carbonization treatment for 3minutes in the carbonization step. When the carbon fiber precursor fiberbundle was taken out from the container, the bundle was once pulledupward in such a way that the bundle was made to pass through guide barsplural times, so that the small tows were pulled and aligned. The carbonfiber precursor fiber bundle thus aligned by pulling was fed to-theflame retarding step without dividing into the small tows.

In the course of these operations, the rolls used for traveling the towwere all flat rolls. Absolutely, neither division into small tows norcontrol of the form of the tow using a roll having a groove on thesurface thereof or the like was carried out. In the flame retardingstep, as the reaction proceeded, division into small tows occurredspontaneously without using dividing guides or the like. The carbonfiber bundle obtained after the carbonization treatment was free fromfuzz and excellent in grade. The strand strength of the obtained carbonfiber was 4900 MPa.

Example 2

As shown in FIG. 4, with a touch roll 16, ion-exchanged water wasimparted to small tows 1, each having 50,000 filaments, obtained in thesame manner as in Example 1, and then each of the small tows 1 wasseparately fed to a first intermingling device 3 shown in FIG. 2. Thefundamental structure of the first intermingling device 3 dedicated to asmall tow was similar to that in Example 1: the yarn channel width was16 mm to be twice as wide as that in Example 1, the yarn channel heightwas 2.5 mm to be slightly smaller than that in Example 1, the yarnchannel length was 20 mm to be the same as that in Example 1, theopening diameter of each of the air jet holes 5 b and 6 b was 1 mm to bethe same as that in Example 1, and the disposition pitch thereof was setat 1.0 mm. The pressure of the fed air was set at 100 kPa-G to be twiceas large as that in Example 1.

Subsequently, the three small tows 1 thus obtained were aligned bypulling and fed to a second intermingling device having a structure asshown in FIG. 5 which device intermingled adjacent small tows 1 witheach other.

The second intermingling device 17 was different from the interminglingdevice 8 shown in FIG. 3 in that the yarn channel 9 had simply a flatrectangular section, but the upper and lower nozzles 18 and 19 of thesecond intermingling device 17 applied to present Example furtherincluded grooves 18 c and 19 c each having a trapezoidal section,respectively, above and below each of the portions in the flatrectangular section which portions corresponded to the abuttingpositions of the three small tows 1 adjacent to each other. The otheraspects of the structure of the second intermingling device 17 weresubstantially the same as those in Example 1. In present Example, thesecond intermingling device 17 had the following dimension: the width ofthe yarn channel 20 was 45 mm to be wider by 21 mm than that in Example1, the yarn channel height was 2.5 mm to be the same as that in Example1, the opening diameter of each of the air jet holes 18 b and 19 b was0.5 mm to be the same as that in Example 1, the disposition pitchthereof was 1.0 mm, the length of the long side of the trapezoidal grovesection was 7 mm, and the length of the short side corresponding to thegroove bottom was 3 mm. The pressure of the fed air was set at 200 kPa-Gto be ⅔ that in Example 1. The carbon fiber precursor fiber bundle 12thus obtained was fed to the gear roll 13 adjunct to a transfer deviceto be placed in a container 15 through a chute 14. In this case, themoisture content of the bundle after being housed was 2% by mass.

The carbon fiber precursor fiber bundle 12, leaving the secondintermingling device 17, had a form of a single tow in which three smalltows 1 were aggregated. By the gear roll 13 adjunct to the transferdevice, wave was imparted to the carbon fiber precursor fiber bundle 12placed in the container 15. The distance between a crest of the wave andan adjacent crest was 25 mm. The degree of intermingle of the carbonfiber precursor fiber bundle thus obtained was evaluated and found to beless than 1 m⁻¹. (The evaluation was carried out with a test length of 1m, and any of the 10 g loads fell with a falling distance of 1 m ormore, so that the measurement was impossible.)

In the same manner as in Example 1, the carbon fiber precursor fiberbundle 12 thus obtained was taken out from the container 15, and was fedto the flame retarding step without dividing into small tows, subjectedto a flame retarding treatment for 70 minutes, and further subjected toa carbonization treatment for 3 minutes in the carbonization step. Inthe course of these operations, the rolls used for traveling the carbonfiber precursor fiber bundle 12 were all flat rolls. Absolutely, neitherdivision into small tows nor control of the form of the tows using aroll having a groove on the surface thereof or the like was carried out.In the flame retarding step, as the reaction proceeded, division intosmall tows occurred spontaneously without using dividing guides or thelike. The carbon fiber obtained after the carbonization treatment wasfree from fuzz and excellent in grade. The strand strength of theobtained carbon fiber was 4900 MPa.

Example 3

There was used a second intermingling device 24 shown in FIG. 6, tointermingle small tows 1 with each other, having the same structure asthat in Example 2 except that a plurality of air jet holes 22 b and 23 bwere formed in the grooves 22 c and 23 c communicatively connected to ayarn channel 21, but no air jet holes were formed in the portions otherthan the grooves. Using this second intermingling device, a carbon fiberprecursor fiber bundle having a form of a single tow in which threesmall tows were aggregated was obtained in the same manner as in Example2. One carbon fiber precursor fiber bundle thus obtained was fed to agear roll 13 to be taken up, and then placed as it was in a container 15through a chute 14. At this time, namely, after being housed in thecontainer, the moisture content was 4% by mass. When housed in thecontainer 15, the carbon fiber precursor fiber bundle 12 had a form of asingle tow in which three small tows 1 were aggregated. At this time,namely, after being housed in the container, the moisture content of thecarbon fiber precursor fiber bundle 12 was 2% by mass. By the gear roll13 used when placed in the container 15, wave was imparted to theobtained tow. The distance between a crest of the wave and an adjacentcrest was 25 mm. The degree of intermingle of the carbon fiber precursorfiber bundle 12 thus obtained was evaluated and found to be less than 1m⁻¹. (The evaluation was carried out with a test length of 1 m, and anyof the 10 g loads fell with a falling distance of 1 m or more, so thatthe measurement was impossible.)

In the same manner as in Example 1, the carbon fiber precursor fiberbundle 12 thus obtained was taken out from the container 15, and was fedto the flame retarding step without dividing into small tows, subjectedto a flame retarding treatment for 70 minutes, and further subjected toa carbonization treatment for 3 minutes in the carbonization treatmentstep.

In the course of these operations, the rolls used for traveling the towwere all flat rolls. Absolutely, neither division into small tows norcontrol of the form of the tow using a roll having a groove on thesurface thereof or the like was carried out. In the flame retardingstep, as the reaction proceeded, division into small tows occurredspontaneously without using dividing guides or the like. The carbonfiber bundle obtained after the carbonization treatment was free fromfuzz and excellent in grade. The strand strength of the obtained carbonfiber was 4900 MPa.

Example 4

A carbon fiber precursor fiber bundle 12 was placed in a container 15through the same intermingling procedures as in Example 3 except that anintermingling device 25 having a structure shown in FIG. 7 was used asthe second intermingling device to intermingle adjacent small tows witheach other. The second intermingling device 25 was the same as theintermingling device of Example 3 (FIG. 6) except that grooves 27 c and28 c, each having a semicircular section the diameter of which was 6 mmand having a grove depth of 3 mm, respectively, were formed above andbelow each of the portions which corresponded to the abutting positionsof the three small tows 1 in the flat rectangular section of a yarnchannel 26; the small tows were intermingled with each other by jettingout air from a plurality of air jet holes 27 b and 28 b in the samemanner as in Example 3.

The degree of intermingle of the carbon fiber precursor fiber bundlethus obtained was evaluated and found to be less than 1 m⁻¹. (Theevaluation was carried out with a test length of 1 m, and any of the 10g loads fell with a falling distance of 1 m or more, so that themeasurement was impossible.)

In the same manner as in Example 1, the carbon fiber precursor fiberbundle 12 thus obtained was taken out from the container 15, and was fedto the flame retarding step without dividing into small tows, subjectedto a flame retarding treatment for 70 minutes, and further subjected toa carbonization treatment for 3 minutes in the carbonization step. Inthe course of these operations, the rolls used for traveling the towwere all flat rolls. Absolutely, neither division into small tows norcontrol of the form of the tow using a roll having a groove on thesurface thereof or the like was carried out. In the flame retardingstep, as the reaction proceeded, division into small tows started tooccur spontaneously without using dividing guides or the like. Thecarbon fiber obtained after the carbonization treatment was perfectlydivided into small tows, free from fuzz and excellent in grade. Thestrand strength of the obtained carbon fiber was 5100 MPa.

Example 5

A carbon fiber precursor fiber bundle was placed in a container 15 inthe same manner as in Example 4 except that a nip roll having a flatsurface was used in place of the gear roll 13 in Example 4. Thereafter,a carbon fiber strand was obtained in the same manner as in Example 4(Example 1).

When housed in the container 15, the carbon fiber precursor fiber bundle12 had a form of a tow in which three small tows 1 were aggregated. Atthis time, the moisture content of the carbon fiber precursor fiberbundle 12 was 2% by mass.

The degree of intermingle of the carbon fiber precursor fiber bundle 12thus obtained was evaluated and found to be less than 1 m⁻¹. (Theevaluation was carried out with a test length of 1 m, and any of the 10g loads fell with a falling distance of 1 m or more, so that themeasurement was impossible.)

In the same manner as-in Example 1, the carbon fiber precursor fiberbundle 12 thus obtained was taken out from the container 15, and was fedto the flame retarding step without dividing into small tows, subjectedto a flame retarding treatment for 70 minutes, and further subjected toa carbonization treatment for 3 minutes in the carbonization step.

In the course of these operations, the rolls used for traveling the towwere all flat rolls. Absolutely, neither division into small tows norcontrol of the form of the tow using a roll having a groove on thesurface thereof or the like was carried out. In the flame retardingstep, as the reaction proceeded, division into small tows occurredspontaneously without using dividing guides or the like. The carbonfiber bundle obtained after the carbonization treatment was free fromfuzz and excellent in grade. The strand strength of the obtained carbonfiber was 4900 MPa.

Example 6

A carbon fiber strand was obtained in the same manner as in Example 1except that the total drawing magnification was set at 9.

Example 7

A carbon fiber strand was obtained in the same manner as in Example 1except that the nozzle hole diameter was set at 75 μm and the totaldrawing magnification was set at 9.

Comparative Example 1

A small tow obtained by the production method (I) of a small tow wasused, and intermingling within the small tow was carried out in the samemanner as in Example 1. Three small tows thus obtained was fed to acrimp-imparting device, which is not shown in the drawings, and bundledby crimping. The bundled tow was housed in a container in the samemanner as in Example 1.

The carbon fiber precursor fiber bundle thus obtained was taken out fromthe container, and was subjected to a flame retarding treatment for 70minutes, and further subjected to a carbonization treatment for 3minutes. When the carbon fiber precursor fiber bundle was taken out fromthe container, the bundle was once pulled upward, in the same manner asin Example 5, in such a way that the bundle was made to pass throughguide bars plural times, so that the small tows were aligned by pulling.The carbon fiber precursor fiber bundle thus aligned by pulling was fedto the flame retarding step without dividing into small tows, subjectedto a flame retarding treatment for 70 minutes, and further subjected toa carbonization treatment for 3 minutes. In the course of theseoperations, the rolls used for traveling the tow were all flat rolls.Absolutely, neither tow division nor control of the form of the towusing a roll having a groove on the surface thereof or the like wascarried out. In the flame retarding step, as the reaction proceeded,division into small tows occurred spontaneously without using dividingguides or the like. However, the carbon fiber obtained after thecarbonization treatment was abundant in fuzz and not excellent in grade.In the flame retarding step, there frequently occurred winding aroundrolls conceivably ascribable to the fuzz. Additionally, the strandstrength of the obtained carbon fiber was 3600 MPa.

1. A carbon fiber precursor fiber bundle, characterized by: having 1 m⁻¹or less of a degree of intermingle between a plurality of small towsbased on the hook drop method; consisting of substantially straightfibers without imparted crimp, a tow of which straight fibers has amoisture content of less than 10% by mass when housed in a container;and having a widthwise dividing capability to maintain a form of asingle aggregate of tows when housed in a container, taken out from thecontainer and guided into a firing step, and to divide into a pluralityof small tows in the firing step by the tension generated in the firingstep;
 2. The carbon fiber precursor fiber bundle according to claim 1,wherein the monofilament fineness is 0.7 dtex or more and 1.3 dtex orless, the number of monofilaments of the small tow is 50,000 or more and150,000 or less, and the total number of monofilaments in the aggregateof tows is 100,000 or more and 600,000 or less.
 3. The carbon fiberprecursor fiber bundle according to claims 1 or 2, wherein the aggregateof tows is formed by intermingling monofilaments between a widthwise endof a small tow and a widthwise end of the adjacent small tow by airflow.
 4. The carbon fiber precursor fiber bundle according to any one ofclaims 1 to 3, wherein the number of monofilaments undergoing adhesionbetween the monofilaments is 5 per 50,000 of monofilaments or less andthe size of the crystal region in a direction perpendicular to the fiberaxis is 1.1×10⁻¹ m or more.
 5. The carbon fiber precursor fiber bundleaccording to any one of claims 1 to 4, wherein the strength of amonofilament is 5.0 cN/dtex or more, and the fineness unevenness (CVvalue) of the monofilament is 10% or less.
 6. The carbon fiber precursorfiber bundle according to any one of claims 1 to 5, wherein the oilingagent adhesion unevenness (CV value) along the lengthwise direction is10% or less.
 7. A production method of a carbon fiber precursor fiberbundle, characterized by comprising: a coagulation step of formingswollen yarn by extruding an organic solvent solution of anacrylonitrile-based polymer into an aqueous solution ofdimethylacetamide from a spinning nozzle having a nozzle hole diameterof 45 μm or more and 75 μm or less and the number of holes of 50,000 ormore at a coagulated yarn take-up speed/extrusion linear speed ratio of0.8 or less; a wet heat drawing step of wet heat drawing the swollenyarn; an oiling agent imparting step of imparting a first oiling agentto the wet heat drawn yarn by guiding the heat wet drawn yarn into afirst oil bath, and subsequently imparting a second oiling agent in asecond oil bath after once squeezing the yarn by use of two or moreguides; a small tow production step of obtaining a small tow by drying,densifying and secondarily drawing the yarn imparted with the first andsecond oiling agents so as to have a total drawing magnification of 5 ormore and 10 or less; and an aggregate-of-tows production step ofobtaining an aggregate of tows by feeding a plurality of the small towsso as to be in parallel and adjacent to each other into an interminglingdevice that comprises a yarn channel having a flat rectangular sectionand a plurality of air jet holes which are disposed with a predeterminedinterval along the long side direction of the flat rectangle and whichopen into the yarn channel, and by jetting out air from the air jetholes to intermingle the adjacent small tows with each other.
 8. Theproduction method of a carbon fiber precursor fiber bundle according toclaim 7, further comprising an aggregate-of-tows housing step of housingthe aggregate of tows in a container after the aggregate-of-towsproduction step and a water imparting step of imparting water to thesmall tows before the aggregate-of-tows production step, wherein thewater content of the aggregate of tows in the aggregate-of-tows housingstep is set at less than 10% by mass.
 9. The production method of acarbon fiber precursor fiber bundle according to claims 7 or 8, furthercomprising, before the aggregate-of-tows production step, anintra-small-tow intermingling step of intermingling monofilaments withinthe small tow with each other by passing the small tow through anintermingling device, other than the intermingling device used in theaggregate-of-tows production step, that comprises a yarn channel havinga circular section and an air jet hole which opens into this yarnchannel, and by jetting out air from this air jet hole.
 10. Theproduction method of a carbon fiber precursor fiber bundle according toclaims 7 or 8, further comprising, before the aggregate-of-towsproduction step, an intra-small-tow intermingling step of interminglingthe monofilaments within the small tow with each other by passing thesmall tow through an intermingling device, other than the interminglingdevice used in the aggregate-of-tows production step, that comprises ayarn channel having a flat rectangular section and a plurality of airjet holes which are disposed with a predetermined interval along thelong side direction of this flat rectangle and which open into this yarnchannel, and by jetting out air from these air jet holes.
 11. Theproduction method of a carbon fiber precursor fiber bundle according toclaims 7 or 8, wherein monofilaments within the small tow areintermingled with each other in the aggregate-of-tows production step.12. The production method of a carbon fiber precursor fiber bundleaccording to claim 11, wherein the intermingling device, used in theaggregate-of-tows production step, further comprises a groove whichextends along the lengthwise direction of the yarn channel and whichopens into the yarn channel at a position where the small tows areadjacent to each other.
 13. The production method of a carbon fiberprecursor fiber bundle according to claims 9 or 10, wherein: theintermingling device, used in the aggregate-of-tows production step,further comprises a groove which extends along the lengthwise directionof the yarn channel and which opens into the yarn channel at a positionwhere the small tows are adjacent to each other, and the air jet holesopen only into the groove; and a plurality of the small tows areintermingled with each other, wherein the filaments within the smalltows are intermingled with each other, by feeding to this interminglingdevice the plurality of the small tows having been subjected to theintra-small-tow intermingling step.
 14. The production method of acarbon fiber precursor fiber bundle according to any one of claims 7 to13, further comprising a step of housing in a container the aggregate oftows obtained in the aggregate-of-tows production step after theaggregate of tows has been fed to a gear roll.
 15. The production methodof a carbon fiber precursor fiber bundle according to any one of claims7 to 13, further comprising a step of housing in a container theaggregate of tows obtained in the aggregate-of-tows production stepafter the aggregate of tows has been fed to a nip roll.
 16. A productionapparatus of a carbon fiber precursor fiber bundle, characterized bycomprising an intermingling device that comprises a yarn channel havinga flat rectangular section capable of passing a plurality of small towswhich are adjacent to each other and that comprises a plurality of airjet holes which are disposed with a predetermined interval along thelong side direction of the flat rectangle and which open into the yarnchannel.
 17. The production apparatus of a carbon fiber precursor fiberbundle according to claim 16, further comprising a groove which extendsalong the lengthwise direction of the yarn channel and which opens intothe yarn channel at a position where the plurality of small tows areadjacent to each other.
 18. A production apparatus of a carbon fiberprecursor fiber bundle, characterized by comprising: a firstintermingling device that comprises a yarn channel having a circularsection capable of passing a small tow and that comprises one or moreair jet holes for jetting out air into the yarn channel; and a secondintermingling device that comprises a yarn channel having a flatrectangular section capable of passing a plurality of small tows whichare adjacent to each other and that comprises a plurality of air jetholes which are disposed with a predetermined interval along the longside direction of this flat rectangle and which open into this yarnchannel.
 19. A production apparatus of a carbon fiber precursor fiberbundle, characterized by comprising: a first intermingling device thatcomprises a yarn channel having a flat rectangular section capable ofpassing a small tow and that comprises one or more air jet holes forjetting out air into the yarn channel; and a second intermingling devicethat comprises a yarn channel having a flat rectangular section capableof passing a plurality of small tows which are adjacent to each otherand that comprises a plurality of air jet holes which are disposed witha predetermined interval along the long side direction of this flatrectangle and which open into this yarn channel.
 20. The productionapparatus of a carbon fiber precursor fiber bundle according to claims18 or 19, wherein the second intermingling device further comprises agroove which extends along the lengthwise direction of the yarn channelthereof and which opens into the yarn channel at a position where theplurality of small tows are adjacent to each other.
 21. The productionapparatus of a carbon fiber precursor fiber bundle according to claim20, wherein the air jet holes of the second intermingling device openonly into the groove.
 22. The production apparatus of a carbon fiberprecursor fiber bundle according to claim 16, wherein the ratio n·D/L ofthe total fineness nD (dTex) of an aggregate of tows represented by theproduct between the total fineness D (dTex) of the small tow and thenumber n of the small tows to be aggregated to the long side dimension L(mm) of the flat rectangular section is 2,000 dTex/mm or more and 12,000dTex/mm or less, and the diameter of each of the air jet holes is 0.3 mmor more and 1.2 mm or less.
 23. The production apparatus of a carbonfiber precursor fiber bundle according to claim 16, wherein the air jetholes are disposed with an even pitch, and the pitch is 0.8 mm or moreand 1.6 mm or less, and the length of the yarn channel is 10 mm or moreand 40 mm or less.
 24. The production apparatus of a carbon fiberprecursor fiber bundle according to claims 17 or 20, wherein the groovehas a sectional shape of a part of a circle, and the diameter of thecircle is 2 mm or more and 10 mm or less, and the depth of the groove is1.5 mm or more and 4 mm or less.
 25. The production apparatus of acarbon fiber precursor fiber bundle according to claims 17 or 20,wherein the groove has a trapezoidal sectional shape, and the dimensionof the long side of the trapezoidal groove section is 2 mm or more and10 mm or less, and the dimension of the short side corresponding to thegroove bottom is 1.5 mm or more and 6 mm or less.
 26. A productionmethod of a carbon fiber, characterized in that the carbon fiberprecursor fiber bundle according to any one of claims 1 to 6 is fed to aflame retarding step, and is fired while being divided into small towsby the tension generated in the flame retarding step.
 27. A productionmethod of a carbon fiber, characterized in that the carbon fiberprecursor fiber bundle according to any one of claims 1 to 6 is fed to acarbonization step after a flame retarding step, and is fired whilebeing divided into small tows by the tension generated in thecarbonization step.
 28. A carbon fiber characterized in that the carbonfiber is produced by the method according to claim 27 and the strandstrength thereof defined by JIS R7601-1986 is 4100 MPa or more.
 29. Aproduction method of a carbon fiber precursor fiber bundle,characterized by comprising a step of obtaining a single aggregate oftows by disposing a plurality of small tows of carbon fiber precursorfiber so as to be in parallel and adjacent to each other, and byintermingling the adjacent small tows with each other by air flow. 30.The production method of a carbon fiber precursor fiber bundle accordingto claim 29, wherein, in the step of obtaining an aggregate of tows, theintermingling is carried out by feeding a plurality of the small tows soas to be in parallel and adjacent to each other into an interminglingdevice that comprises a yarn channel having a flat rectangular sectionand a plurality of air jet holes which are disposed with a predeterminedinterval along the long side direction of the flat rectangle and whichopen into the yarn channel, and by jetting out air from the air jetholes.